The present invention relates to power factor correction circuits, particularly for use with AC-to-DC converters.
Most off-line switched mode power supplies (SMPS) use a two stage approach in converting an AC input voltage into a DC output voltage. The first stage generally converts the unregulated AC input voltage into an unregulated and partially filtered DC voltage, the second stage then generates the regulated DC voltage.
The first stage is an AC-to-DC converter generally consisting of a full-wave, or a full- wave bridge, rectifier followed by a bulk capacitance to store and deliver energy to the second stage. Because the rectified line voltage is sinusoidal, its value is below that of the capacitor voltage over most of the AC line cycle period. Thus, when the capacitor delivers power to the second stage it must recover that energy in the relatively short time the AC input voltage exceeds the capacitor voltage. As a result, huge impulses of a current are drawn from the line. This not only reduces the power factor but also increases the stress on the rectifiers and capacitor and pollutes the line with harmonics. It is desirable to operate a switched mode power supply source to maximize the use of available power from the AC voltage mains as well as eliminate the harmonic content conducted, in other words, to operate the power supply at unity power factor. This can be achieved by forcing the input current to be sinusoidal.
The prior art does contain circuits which have attempted to provide near unity power factor in switched mode power supplies. For example, U.S. Pat. No. 4,437,146 (Carpenter) discloses a power factor correction circuit in which a shunting switch is provided for the purpose of causing the input current to follow a sinusoidal waveform in phase with the input voltage. A current sense amplifier is used to sense the input current. An error amplifier is provided to generate a feedback voltage signal responsive to the deviation of the voltage at the output of the power supply from a predetermined reference. The outputs of the current sense amplifier and the error amplifier are used in controlling the switching of the shunting switch. A drawback of the disclosed circuit in the Carpenter patent is that it does not disclose a means for quickly responding to variations in load.
It is an object of the present invention to provide a power factor correction circuit with fast response to load as well as input voltage variations, by means of a relatively high bandwidth feedback loop and by feedforward of the input voltage respectively.